Bugfix Input use std::string (#279)

Use std::string by default for the implementation of FTXUI's input
component.

Along the way:
- Give a correct implementation for fullwidth characters.
- Add tests
- Modify the way the cursor is drawn.

include/ftxui/screen/string.hpp

@@ -14,7 +14,20 @@ std::wstring to_wstring(T s) {
 }
 
 int string_width(const std::string&);
+// Split the string into a its glyphs. An empty one is inserted ater fullwidth
+// ones.
 std::vector<std::string> Utf8ToGlyphs(const std::string& input);
+// If |input| was an array of glyphs, this returns the number of char to eat
+// before reaching the glyph at index |glyph_index|.
+int GlyphPosition(const std::string& input,
+                  size_t glyph_index,
+                  size_t start = 0);
+// Returns the number of glyphs in |input|.
+int GlyphCount(const std::string& input);
+
+// Map every cells drawn by |input| to their corresponding Glyphs. Half-size
+// Glyphs takes one cell, full-size Glyphs take two cells.
+std::vector<int> CellToGlyphIndex(const std::string& input);
 
 }  // namespace ftxui